1. Field of the Invention
The present invention relates to an image forming apparatus capable of operating to form an image both in a horizontal installation and in a vertical installation of the apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile, etc., a latent image forming apparatus like an electrophotographic apparatus has been utilized from a demand for recording on ordinary paper. In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive drum. Next, the electrostatic latent image on this photosensitive drum is developed with a powder developer and is thereby transformed into a visible image. Further, the powder developer on the photosensitive drum is transferred onto a sheet. This sheet is separated therefrom, and the powder-developed image on the sheet is fixed.
In this type of image forming apparatus, it is desirable to make effective use of an installation space by changing an installation position of the apparatus.
FIG. 13 is a view of assistance in explaining a prior art.
In the electrophotographic apparatus, it is required that feeding of the sheet be synchronized with a position of a toner image on a photosensitive drum 90 to transfer the toner image on the photosensitive drum 90 onto the sheet. For this purpose, the sheet taken out of a sheet cassette impinges on a resist roller 91 and is thereafter fed by the resist roller 91 in synchronism with the toner image on the photosensitive drum 90.
A sheet detector 8 is provided for detecting that the sheet reaches this resist roller 91. In general, this sheet detector 8 involves the use of a known photosensor. In the image forming apparatus, however, the toners are easy to scatter within the apparatus. For this reason, the scattered toners are adhered to the photosensor, resulting in a decline in terms of a performance of the sensor. Therefore, a mechanical sensor for mechanically detecting the sheet is employed.
As illustrated in FIG. 13, this type of sheet detector 8 includes a detection lever 82 secured to a rotary shaft 81 provided in a lever holding block 80. A front end of the detection lever 82 is protruded into a feeding path defined by a couple of sheet guides 93, 94. On the other hand, a transmission type sensor 83 is provided in the lever holding block 80, corresponding to a rear end of the detection lever 82. Further, the detection lever 82 is biased in a direction of the feeding path by a spring 84.
An operation of the construction shown in FIG. 13 will be explained. In a state where the sheet does not exist in the feeding path, as illustrated in FIG. 13, the rear end of the detection lever 82 does not intercept the transmission type sensor 83, and, therefore, the non-existence of the sheet can be recognized from an output of the transmission type sensor 83. On the other hand, if a leading edge of the sheet touches the front end of the detection lever 82, the detection lever 82 rotates clockwise in the FIGURE by dint of a sheet feeding force, resisting the biasing force of the spring 84. With this action, the rear end of the detection lever 82 intercepts the transmission type sensor 83, whereby reaching of the sheet can be detected. In this mechanical sensor, the biasing force of the spring 84 is set smaller than the sheet feeding force.
On the other hand, for instance, Japanese Patent Laid-Open Publication Number 4-323125 discloses an image forming apparatus which operates even when installed horizontally or vertically. This image forming apparatus aims at operating to form the image both in the horizontal installation and in the vertical installation of the apparatus.
According to the prior arts, however, first, a force of rotary moment varies depending on a position of center-of-gravity of the detection lever when the apparatus is installed vertically and when installed horizontally in the above-mentioned sheet detector. For this reason, a detection timing of the sheet differs when the apparatus is installed horizontally and when installed vertically. This conduces to an occurrence of a deviation of the transferred image because of a difference in terms of a drive timing of the resist roller.
Secondly, when attaining down-sizing of the apparatus, the above sheet detector also decreases in size. The spring constituting this detector becomes small on the order of, e.g., several mm. Accordingly, it takes a good deal of labors to attach such micro parts, resulting in an increase in costs for the apparatus.